Question: write the ratio $15:42$ in the form $1 : n$
1. **State the problem:** We need to write the ratio $15:42$ in the form $1 : n$.
2. **Formula and rule:** To express a ratio $a:b$ as $1:n$, divide both terms by $a$.
3. **Apply the formula:**
$$\frac{15}{15} : \frac{42}{15} = 1 : \frac{42}{15}$$
4. **Simplify the fraction:**
$$1 : \frac{42}{15} = 1 : \frac{\cancel{42}}{\cancel{15}} = 1 : \frac{14}{5}$$
5. **Convert to decimal (optional):**
$$1 : 2.8$$
**Final answer:** The ratio $15:42$ in the form $1 : n$ is
$$1 : \frac{14}{5}$$
or approximately
$$1 : 2.8$$